Why do we use a frequency of 50 Hz

Why do we have the frequency 50 Hz today?

First luminous flux with "16,000 changes per minute": 133 Hz

In 1886 the world's first alternator was built by the electrical pioneer George Westinghouse in the USA [1]. The generator produced a current of "16,000 changes per minute". Because every two changes form a full period, this corresponded to a value of 16,000 / (2 x 60) = 133 periods per second, i.e. the frequency 133 Hz in today's perception.

How could this strange numerical value arise? For the test machine, George Westinghouse had taken a dynamo magnet frame with eight poles from his stock of types and built a cylindrical rotor for it, on the surface of which he attached electrical wires. This “winding” inevitably had a small number of conductors. In order for it to produce a usable induction voltage, the rotor had to be rotated fairly quickly in the magnetic field. Thanks to a belt drive with a high-speed ratio, 2000 revolutions per minute were achieved, and this resulted in the aforementioned 2000 x 8 = 16,000 pole changes per minute, i.e. the "crooked" 133⅓ Hz. - The competitor Thomson-Houston (later General Electric after merging with the Edison General Electric Company) preferred 15,000 pole changes, which corresponded to a frequency of 125 Hz.

At the time when 133 Hz and 125 Hz current were first used, there were no induction motors, no electrical power transmissions and no parallel operation of generators, which would have required special consideration with the frequency. Actually, it was all about electric light. In America, “light distribution systems” were common, in which each feed line was supplied from a control center by its own generator and each consumer was connected directly to the primary network via a small transformer. High frequencies were an advantage here, as they made it possible to use transformers that were light in weight and could easily be hung from the electricity pylon.

Power current with low alternation rate: 25 Hz and more

High numbers of periods soon turned out to be an obstacle in the construction of rotating electrical machines that were to be used as motors, generators and converters. In order to be able to drive with short tours, a rather small number of poles had to be taken for structural reasons, which also corresponded to a low frequency. Examples: The generators of the Lauffen am Neckar hydropower plant (Fig. 1) from 1891 had 150 tours per minute and 32 poles; accordingly, they generated a 40 Hz current (150 x 32/2 / 60 = 40). The Niagara generators from 1895 rotated at 250 revolutions per minute; with 12 poles this resulted in 250 x 12/2 / 60 = 25 Hz. The trend towards low speeds was intensified by the transition to direct coupling with water turbines or piston engines.

Luminous flux not below "5,000 changes per minute": 41 Hz (42 Hz)

In the meantime, the Hungarian electrical company Ganz had systematically investigated the problem of light twitching in electric arc lamps, which was due to the low number of periods in alternating currents. It was found that the human eye perceived the light as calm whenever the current changed 5000 times or more per minute. The company then set a lowest turnover rate of 5000 for its products. This corresponds to the frequency 41⅔ Hz or around 42 Hz. In the sales areas of the Ganz works, including Italy, the 42 Hz frequency was very common and in use for decades.

North America's Path to 60 Hz (and 25 Hz)

In the USA, too, where alternating current technology had its beginnings for light with high cycle numbers, it soon had to be recognized that new applications of electricity required lower alternation numbers [1]. So one went over to lower frequencies on a case-by-case basis, and over time a colorful range of utility values ​​emerged:

133⅓ Hz (16,000 changes), 125 Hz (15,000), 83⅓ Hz (10,000), 66⅔ Hz (8,000), 60 Hz, 50 Hz, 40 Hz, 30 Hz, 25 Hz and many others .

As early as the early 1890s, the Westinghouse Company and its star consultant Nikola Tesla recommended the use of 7200 changes per minute (60 cycles / second) for new systems, which was seen as a viable compromise between the conditions of power generation and application. For electrical power transmissions, however, an even lower number of cycles than necessary was found; Incidentally, also for converter machines, which were in demand in many places to supply direct current consumers. For example, a second standard frequency was allowed in the USA and the 25 cycles of the Niagara power plant were adopted for it. - In North America, as is well known, the 60 Hz should finally prevail as the standard frequency for all purposes. The use of 25 Hz was restricted to the US-American Niagara border region, but locally survived the whole of the 20th century.

Europe's way to 50 Hz

In England, the frequencies chosen were initially quite high, similar to the USA. On the European mainland, on the other hand, rather low period numbers were used from the beginning, in Germany mainly those between 40 and 70. Figure 2 shows frequency statistics derived from an overview of the AC power plants newly built in Germany from the period 1891-1901 [2]. One recognizes: 50 Hz was just one frequency among many others between 40 and 70 Hz in Germany at the beginning, but it already took precedence towards the middle of the decade, put other frequencies - namely 25 and 42 Hz - in the corner and was at the turn of the century only choice for new systems.

In Europe, two electrical companies are particularly committed to the 50 Hz frequency: Maschinenfabrik Oerlikon MFO in Switzerland and Allgemeine Elektricitäts-Gesellschaft AEG in Berlin. Both companies worked closely together and had already demonstrated the spectacular high-voltage three-phase current transmission from Lauffen am Neckar (Fig. 1) over a distance of 175 kilometers to the International Electrotechnical Exhibition in Frankfurt am Main in 1891. The transmission was still carried out at a frequency of 40 Hz, final tests with increased power and voltage even took place with a frequency reduced to 24 Hz. As you can see, the frequency was a free parameter in early heavy current engineering, so to speak a "set screw" for system adaptation.

Thanks to this trick, the MFO used its “generator model Lauffen” with 150 rpm and 40 Hz for a next project by increasing the speed and frequency by 25%: In 1892 “Lauffen-like” hydrogen generators went into operation in Hochfelden in northern Switzerland rotated at 187.5 rpm and thus generated a 50 Hz current. It became common practice that, in response to new inquiries, an existing machine model was simply adapted to a slightly different speed without any design changes. The only consequence: a new frequency was introduced!

The MFO can claim that with the Hochfelden delivery, it was presumably the first 50 Hz three-phase generators to be launched. Generators of this type were soon to follow for the Bremgarten power plant on the Reuss. The really big hour for 50 Hz came with the hydropower project on the German bank near Rheinfelden (Fig. 3). The experts were challenged to determine a “most favorable number of changes of electricity for the special case”. Emil Rathenau, General Director of AEG, said in a lecture at the 4th annual meeting of the VDE 1896 in Berlin [3]: "... After detailed considerations, it was decided to use 50 periods per second, because with this number of changes the voltage drop [ the transmission lines] must be kept within reasonable limits by self-induction; It appears to be particularly suitable for the operation of transformers, motors and light bulbs, and the use of arc lamps is also permissible if the requirements for the stability of the light are not exaggerated. "

AEG consistently continued the construction of municipal power plants with 50 Hz. By 1897 alone, power plants were built in Strasbourg, Magdeburg, Plauen, Berlin-Oberspree and Gleiwitz. The 50 Hz wave hit more and more European countries and did not necessarily remain an exclusive product for German and Swiss manufacturers. By the turn of the century, 50 Hz had become something of a common standard.

The standard frequency is 50 Hz

The promotion of the change number 50 to the standard frequency was taken on by the Association of German Electrical Engineers (VDE) after it had formed a standard parts commission for electrical machines and transformers in 1900. The process should be arduous and take three decades [4, 5]. In the first VDE machine standards, edition 1902, the frequency was expressly included in the set of rules, but only in the appendix in the form of the recommendation: The frequency should be 25 or 50. Frequency variant 25 apparently had little interest in the period that followed and was dropped in the next edition, in 1912. It says there: The frequency should be 50. This was considered a “recommendation for new systems and price lists”. The First World War forced an interruption of the standardization work, the stronger was the pressure for standardization afterwards. Switzerland, which from 1918 built a “federal busbar from Lake Constance to Lake Geneva” with a standardized power system, increasingly enjoyed the advantage of a real nationwide 50 Hz standard. The Electrotechnical Association in Vienna decided in 1920 that the "number of periods of 50 per second" should be the norm for Austria. In Great Britain, 50 cycles per second became the national standard in 1925. In Germany, the last word was spoken in 1928, the revised set of rules came into force at the beginning of 1930 and stated that the standardized nominal frequency is 50 Hz.

It should be added that the "European" 50 Hz frequency not only "sets the pace" in the continental European network, which now has 22 countries, but is also accepted as the norm in 4½ continents, while the 60 Hz standard in North America, part of South America and some countries in Asia.

Why exactly the numerical value 50?

There are suspicions that the number 50 was also preferred in Europe because it belongs to a series of norms. Developed by the French military engineer Charles Renard in the 1870s, the "Renard series" had increasingly become the guideline for standardization work. And indeed: within the 10-step decimal geometric standard number series R10

10 / 12,5 / 16 / 20 / 25 / 31,5 / 40 / 50 / 63 / 80 / 100

the number 50 occurs explicitly (however not 60). The author was unable to confirm whether such considerations (standard number instead of free value) were really involved when determining the 50 Hz frequency. But maybe there are further information from the VDE members?


[1] Lamme, Benjamin Garver: The technical story of the frequencies. In: AIEE Transactions 37 (1918) pp. 65-85

[2] Jäger, Kurt: AC power plants in Germany. The transition from direct to three-phase current. VDE Verlag History of Electrical Engineering, Vol. 5, 1987

[3] Rathenau, Emil: The power transmission works in Rheinfelden. In: ETZ Elektrotechnische Zeitschrift 17 (1896) 27, pp. 402-409

[4] Neidhöfer, Gerhard: The way to the standard frequency 50 Hz. How the standard frequency 50 Hz emerged from a jumble of period numbers. In: Bulletin SEV / AES 99 (2008) 17, pp. 29−34

[5] Neidhöfer, Gerhard: 50 Hz frequency. How the standard emerged from a European jumble. In: IEEE Power & Energy Magazine 9 (2011) 4, pp. 66−81